The more scientists study the immune system, the more refinements they discover about the roles of the various white blood cells in identifying invaders and responding to them. This knowledge can be applied to modulating the immune system so it can recognize threats from inside the body, such as cancer; or in convincing it not to attack the body, as in autoimmune diseases.

Alternatively, molecules produced by the immune system can be used for these purposes in a passive therapy. Rituxan, for example, was originally approved to treat B-cell non-Hodgkin's lymphoma, by killing B-cells. It's also used in autoimmune disorders like rheumatoid arthritis, which results in B-cell dysfunction.